FIELD The present disclosure relates to a method of forming sheet metal material, and to a method of preparing sheet metal material for forming.

BACKGROUND

Sheet material is used in the manufacture of high-strength and light-weight panels. The panels may take a complex shape and are used as parts for a vast number of applications, such as in the manufacture of vehicles and aeroplanes.

A piece of sheet material may be cut to a desired shape to define a "blank". The design of a blank may be complex and irregular. The blank is then formed into the final panel. One way to form the blank is to use a die and punch.

In a hot stamping process, the blank is heated to a desired temperature before forming. The heating rate affects the efficiency of production, the properties of the blank during forming and the properties of the final formed panel. Heating a blank is typically done using a furnace, which can be slow, inefficient and is environmentally unfriendly.

Resistance heating has been proposed to accelerate the heating process of metal material. In such processes, a current is passed through the blank, which is heated due to resistive power loss.

However, the process does not provide uniform heat to the blank which means the resulting effect is inconsistent, if the current density is not uniform. Accordingly, there is an ongoing desire to improve the heating step of the component forming process to provide consistent results at speed and with minimal inefficiency.

Additionally, during forming, strains occur in the material. This can lead to buckling of the material, cracks and wrinkling. In order to counteract these effects, it is common to apply significant holding forces to maintain the blank in in a preferred position, but these holding forces can themselves cause thinning and cracks. As a result, there is a continuing need to improve the forming process. SUMMARY

In a first aspect there is provided a method of preparing a sheet metal material for forming, the method comprising: preparing the sheet metal material to have a desired resistance profile along an axis; heating the sheet metal material to a desired temperature by passing a current along the axis; and cutting the sheet metal material to define a blank for forming into a predefined arrangement.

In a hot forming, the sheet metal material must be at a desired temperature during the forming stage. The desired temperature depends on the shape and the desired properties of the final panel, the material itself and the process of forming. The desired temperature may be very high. The first aspect can apply this heating to a shape of sheet material designed to optimize the heating process, before subsequently cutting the sheet to define the blank. Thus, the final form of the blank does not impinge on the heating process.

The first aspect takes advantage of the fact that the rate at which an object is heated under resistance or inductance heating will be directly proportional to the resistance of that object. Thus, by maintaining a desired resistance profile, a desired heating profile is also established. This contrasts with heating the blank, which will have a resistance profile which does not take account of a desired heating profile. The resistance profile depends on the shape of the object together with its material properties (i.e. its resistivity). For a given sheet material of uniform material properties, the shape of the material is the deciding factor.

Unlike conventional approaches in which the sheet material is cut into a blank prior to heating, the first aspect requires the sheet metal material is heated to the desired temperature before the sheet metal material is cut to define a blank. Therefore the shape of the sheet metal material during heating is not constrained by the shape of the blank. The shape of the sheet metal material can therefore be chosen according to the heating requirements for forming, rather than according to the shape of the blank required for the final panel.

After heating, the sheet metal material is cut along a boundary to define a blank. The blank is used to form the final component having a predefined arrangement, and the remaining sheet metal material is discarded or recycled. The first aspect can allow for heating techniques such as resistance heating to be used to efficiently heat the sheet metal material to the desired temperature for forming. Optionally, the desired resistance profile is a constant resistance profile along the axis. Sheet metal material has uniform thickness, and a square of a rectangle shape will result in a constant resistance profile along an axis. A constant resistance profile results in a constant heating rate such that the sheet metal material is heated uniformly to the same temperature in a period of time. Further, having a constant resistance profile requires simple shapes which are easy to cut and easy to tessellate. The shapes blanks are cut to in the prior art are complex, and often require lasers. Cutting the sheet metal material to a simple shape does not require lasers. It may be that sheet metal material is created and dispatched in simple shaped form, such as squares or rectangles, for easy shipping and transportation. Using a sheet metal material with a constant resistance profile along an axis can therefore make shipping ready-to-heat sheet metal material easier.

Alternatively, the desired resistance profile is a varying resistance profile along the axis. The resistance profile along an axis can be customized to the temperature requirements of the sheet metal material for forming the panel. If, due to the requirements during the forming process, one section of the sheet metal material needs to be heated to a different temperature than another section of the sheet metal material, the material can be prepared such that the resistance profile is different in different sections. The sections themselves may have constant resistance profiles, leading to a stepped shape of sheet metal material. This way graded temperature fields can be created. Optionally, the method may further comprise cooling the sheet metal material to a second desired temperature. The sheet metal material may be heated to an initial desired temperature then cut along a boundary to define a blank. The initial temperature may be selected to be suitable for cutting the sheet metal material. The sheet metal material may then be cooled to a second, lower desired temperature. The second temperature may be selected to be suitable to forming the sheet metal material into a predefined arrangement or to determine the properties of the sheet metal material once formed into the predefined arrangement.

Optionally, the method may further comprise forming the sheet metal material into a predefined arrangement. The predefined arrangement comprises a three-dimensional shaping of the sheet metal material. Once the sheet metal material has been formed into the predefined arrangement it is the end part, the panel. The panel may be used as part of a vehicle, or machinery, or housing. It will be understood that there are any number of uses of a panel, wherein each use requires the sheet metal material to be arranged in a different three-dimensional shape. Optionally, forming the sheet metal material comprises manipulating the sheet metal material in a direction perpendicular to the plane of the sheet. The sheet metal material is initially flat. It is bent or manipulated in the plane perpendicular to the sheet, such that it forms the final panel. There are a number of known ways of forming sheet metal material.

Optionally, forming the sheet metal material is performed using a tool apparatus. A tool apparatus typically includes a punch and a die which are customized to a mold of the predefined arrangement. The sheet metal material may be pressed into the die during forming, such that the sheet metal material takes the molded shape of the die, i.e. the predefined arrangement.

Optionally, cutting the sheet metal material comprises cutting the sheet metal material to define a boundary of a blank, the boundary comprising at least a section in which separation between the blank and the remaining sheet metal material is incomplete. This can facilitate material flow during forming, reducing wrinkling of the sheet metal material and suppressing buckling. Optionally, forming the material comprises forming the sheet metal material into a predefined arrangement such that separation between the blank and the remaining sheet material is complete.

The sheet metal material could be boron steel, aluminum alloy, magnesium alloy, titanium alloy. According to a second aspect, there is provided a method of forming a sheet material, the method comprising: cutting the sheet material to define a boundary of a blank, the boundary comprising at least a section in which separation between the blank and the remaining sheet material is incomplete; and forming the sheet material into a predefined arrangement, wherein the step of forming causes separation between the blank and the remaining sheet material to be complete.

The sheet material may be a sheet metal material, such as boron steel, aluminum alloy, magnesium alloy, titanium alloy, or alternatively the sheet material may not be a metal material, and may instead be thermoplastic. When a sheet material is formed, it is manipulated out of its original shape and into the predefined arrangement to form the final panel. This is known as "drawing" the sheet material into the predefined arrangement. Conventionally, forming and cutting processes are entirely independent, with the blank being cut from the sheet material before heating, and the forming step occurring after heating. During forming, stresses in the material lead to wrinkling in the material and to buckling of the material. According to the second aspect, the blank is still partially attached to the remaining sheet material when forming begins, and the process of forming itself completes the separation of the blank from the remaining material. This provides extra stresses in the material, tensile stress along the drawing direction and counteracting radial stress, which reduce wrinkling and suppress buckling, and facilitate material flow.

During forming, as the sheet material is formed into the predefined arrangement, the blank is detached from the remaining sheet material and the separation is complete. The remaining sheet material may be discarded or recycled.

Optionally, cutting the material comprises cutting at least partially through the depth of the sheet material. Optionally, cutting the material comprises cutting at least partially along the boundary of the blank. That is, at some instances along the boundary the blank may be completely separated through the thickness from the remaining sheet material, whereas at other instances along the boundary the blank may be attached to the remaining sheet material.

The type of incomplete cut can cause different effects to the stress states of the sheet material. Therefore the type of incomplete cut can be selected to tune the material flow to have a desired effect dependent on the material and on the predefined arrangement. The extent of the incomplete cut (i.e. the extent to which the cut separates the blank from the remaining sheet material) is decided considering the benefit the residual material can provide in stress to reduce wrinkling and suppress buckling. The residual material can be detached form the blank before the forming process is complete. A combination of cutting partially through the depth of the sheet material and cutting partially along the boundary of the blank may be used. A design of complete cut and incomplete cut along the boundary of the blank can be chosen.

Optionally, forming the sheet material comprises manipulating the sheet material in a direction perpendicular to the plane of the sheet. The sheet material is shaped into a three dimensional panel of a shape suitable for the final use of the panel.

Optionally, forming the sheet material is performed using a tool apparatus. A tool apparatus typically includes a punch and a die which are customized to a mold of the predefined arrangement. The sheet material may be pressed into the die during forming, such that the sheet material takes the molded shape of the die, i.e. the predefined arrangement.

Optionally, cutting the sheet material is performed using cutting means attached to the die apparatus. This provides a single apparatus which both cuts and forms the material. Separate equipment for cutting and forming the material is not required, simplifying the procedure through reducing cost and removing the need to transfer the sheet material between the cutting apparatus and the forming apparatus. The method can be performed more efficiently using a single apparatus. Optionally, the cutting and forming are performed simultaneously in a hybrid process. The sheet material does not need to be cut into a blank before the process of forming. Therefore, regular shapes, for example the shape of sheet metal material as distributed in bulk, can be used to create the predefined arrangement. Optionally, forming the sheet material continues after cutting is complete. Forming the sheet material into the predefined arrangement may take a longer time than cutting the material. The forming may therefore continue after the cutting is complete. The forming continues until the sheet material is formed into the predefined arrangement and the blank is completely separated from the remaining sheet material.

Optionally, the sheet material may be sheet metal material, and the method may further comprise, prior to cutting the sheet material, preparing the sheet material to have a desired resistance profile along an axis; and heating the sheet material to a desired temperature by passing a current along the axis. Heating a material alters the properties of the material, and the material may be heated to obtain the desired properties for cutting and/ or forming the material. The resistance profile of the material along one axis may be chosen to obtain the desired temperature when heating via resistance heating. Because the sheet material is cut to form a blank in the method of forming, there is no limitation on the shape of the material when heated. Therefore, the material shape can be chosen based on the requirements for heating rather than the requirements for the final shape of the blank.

According to a third aspect, there is provided an apparatus for forming sheet material, the apparatus comprising: cutting means for cutting the sheet material to define a boundary of a blank, the boundary comprising at least a section in which separation between the blank and the remaining sheet material is incomplete; and forming means for forming the sheet material into a predefined arrangement such that separation between the blank and the remaining sheet material is complete.

A single apparatus can be used to both cut and form a sheet material. This reduces equipment investment for forming a sheet material into a predefined arrangement. Further, it simplifies the process, reducing the time and energy required to complete the process. The sheet material need not be transferred between separate cutting and forming means. This also improves the accuracy of the forming process, as the shape of the blank is exactly aligned with the predefined arraignment. Optionally, the forming means is a tool apparatus. Tool apparatuses typically include a set of tool parts with the predefined arrangement. During forming, the sheet material is drawn into the die. Typically, a tool apparatus includes a punch on one side of the sheet material and a die on the other side of the sheet material. The punch moves towards the die and pushes the sheet material into the die. The sheet material takes the shape of the predefined arrangement.

Optionally, the cutting means is a blade. The blade can be used to cut into the sheet material along the boundary of the blank. The blade can be used to cut partially through the thickness of the sheet material, or to cut completely through the thickness of the sheet material in a section of the boundary.

The blade may be sized such that it is not large enough to cut through the thickness of the sheet metal material. The cutting means may be two blades, arranged to cut from either side of the sheet material. The blade may be attached to the die, or to the blank holders, or to both, such that moving the die and blank holder together moves the blade to cut the sheet material, and also draws the material into the predefined arrangement. Therefore, both cutting and forming can be performed with only one movement. It may be possible to retro-fit cutting means on to an existing die apparatus. Alternatively, the cutting means could be included in the die apparatus on manufacture of the die apparatus.

The sheet material typically has a high surface area to volume ratio.

In a further aspect there is provided a panel obtained by the methods outlined above. It will be appreciated that each of the features described above may apply to each aspect described. All possible combinations are not listed in detail here for the sake of brevity.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described below by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a process diagram illustrating methods for hot forming sheet metal material;

Figures 2a and 2b illustrate sheet metal material with a constant resistance profile along an axis and a varying resistance profile along an axis respectively;

Figure 3 illustrates a die apparatus for cutting and forming sheet material;

Figures 4a to 4c illustrate means for cutting sheet material; Figure 5 illustrates stress in sheet material during forming; and

Figure 6 illustrates a pattern of complete and incomplete cutting along the boundary of a blank in sheet material. SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

With reference to Figure 1, two methods of cutting and forming sheet metal material are shown.

Both methods start with a piece of sheet metal material. The sheet metal material is "as-delivered" to the manufacturer.

The method in the upper arm of Figure 1 is a conventional method. Step 1 of the conventional method is "blanking". That is to cut the as delivered sheet into a desired shape. The sheet metal material is to be used to form a panel of a certain shape and configuration. The panel is made from a shaped section of the sheet metal material (the blank) formed into a three dimensional configuration. Therefore the sheet metal material is cut to define a "blank" required for the final panel. The shape of the blank may be complex and intricate. Due to the complex shape and the high strength of the material, laser cutting is often used which can be slow and expensive. During blanking, the sheet metal material is cut along a boundary to separate the blank from the remaining sheet metal material.

In step 2, the blank is heated to a desired temperature. The desired temperature depends on the properties required for forming the blank and the properties of the final panel. The desired temperature may be high. Forming the blank may involve bending and manipulating the blank in multiple directions and planes, and may require precise manipulation of the blank. The properties of the blank are dependent of the temperature of the blank. The process of forming the blank into the predefined arrangement will require very specific material properties to achieve the desired effect. Therefore the desired temperature for forming is chosen based on the required properties of the blank for forming on the predefined arrangement. The temperature the blank is heated to will also affect the properties of the blank once formed into the predefined arrangement.

Heating is typically done using a furnace. Furnace heating is inefficient and, because blanks can be big and the desired temperature is high, the furnaces are required to be large. Therefore heating requires, for example, a large scale industrial furnace, and involves a large equipment investment. Obtaining access to a suitable furnace may be difficult. The blank may have to be transported to a different location (to the industrial furnace) than the location at which the remaining steps are performed. Furnace heating is typically bad for the environment. The heating rate can affect the properties of the sheet metal material during forming and of the final panel. It can be difficult to finely tune the heating rate in a large industrial furnace which is typically used for many different heating applications. Further, it can be difficult to uniformly heat a blank of a complex shape. It is not feasible to heat the blank using resistance heating. In resistance heating, a current is applied along an axis of an object, and the resistance of that object causes it to heat. A blank will typically have a complex shape, such that the resistance profile along an axis will be very varied. Therefore resistance heating of a blank would lead to non-uniform and inhomogeneous heating. As the shape of the blank is defined by the final panel, it is not possible to customize the shape to achieve a desired temperature. That is, the temperature of the resistance heated blank is constrained by the shape of the final panel, such that resistance heating is not suitable for heating the blank.

In step 3 of the conventional method, the heated blank is formed in to the final panel. The blank is drawn or manipulated along an axis perpendicular to the plane of the sheet into a predefined arrangement to create the final panel.

The method in the lower arm of Figure 1 is a method according to an aspect of the present disclosure.

In step 1, the sheet metal material is cut into a simple shape. The simple shape is chosen to provide the sheet metal material with a desired resistance profile. The desired resistance profile is chosen based on the heating requirements of the sheet metal material for the later forming step. The simple shape will typically have a constant resistance profile along an axis, or a stepped resistance profile along an axis. Simple cutting does not require laser cutting.

It may be that the as delivered sheet metal material has a constant resistance profile along an axis and can be used for the remaining steps of the process without needing to be cut into a simple shape.

In step 2, the sheet metal material is heated using resistance heating to a desired temperature. A current is applied along an axis of the sheet metal material. The resistance of the sheet metal material along that axis causes the sheet metal material to heat up. The amount of heat generated is dependent on the resistance along the axis. The heating is fast and efficient and does not require large equipment investment. Resistance heating can be done at the same location as the other steps of the process.

It will be appreciated that the desired resistance profile of the simple shape cut in step 1 can be determined based on the desired temperature for forming the sheet metal material. The resistance profile of the sheet metal material determines the temperature the sheet metal material will be heated to. Accordingly, the resistance profile must be determined based on the desired temperature, and therefore also based on the predefined arrangement. The desired temperature may be selected to determine the properties of the sheet metal material during forming or to determine the properties of the sheet metal material on the sheet metal material has been formed into the predefined arrangement.

In step 3, the heated sheet metal material is cut into a blank and formed into a predefined arrangement to create the final panel. The steps of cutting and forming may be done

simultaneously. The remaining sheet metal material which is not part of the blank can be discarded or recycled. In this process, all of the sheet metal material, including the blank and the remaining sheet metal material is heated. The sheet metal material may be boron steel, aluminum alloy, magnesium alloy or titanium alloy.

Figure 2a shows a piece of sheet metal material to be heated to a uniform temperature. The sheet metal material 10 includes a blank 12. The blank is the material which will eventually become the final panel. The blank has a boundary 14. The remaining sheet metal material 16 will not form part of the final panel.

Sheet metal material typically has a constant thickness. The sheet metal material 10 in Figure 2a is a rectangle with constant width, and therefore has a constant resistance profile along the x axis. During resistance heating, for example in the fast heating stage of the method of Figure 1, current is passed along the x axis. The resistance along the axis cases the sheet metal material to heat. As the resistance profile is constant along the x axis, the sheet metal material 10 is heated to a uniform temperature.

Figure 2b shows a piece of sheet metal material 10 to be heated form forming. Like reference numbers are used to show like parts. The desired temperature for forming the sheet metal material 10 includes different temperatures in different areas of the sheet metal material. This may be because different post-form mechanical properties are required for applications. The left hand side of the blank is required to be at a higher temperature for forming than the right hand side of the blank.

The sheet metal material has been cut and prepared to have a resistance profile based on the desired temperature for forming. The sheet metal material 10 has two different widths (a smaller width on the right hand side and a larger width on the left hand side). The resistance profile of the sheet metal material 10 along the x axis is therefore not constant. The left hand side has a higher resistance than the right hand side. During resistance heating, current is passed along the x axis. The section with the higher resistance heats faster than the section with the lower resistance, such that the left hand section ends up being hotter than the right hand section.

It is apparent that the shape of the sheet metal material 10 can be chosen to tune the temperature of the sheet metal material 10, and the blank 12, for forming.

Figure 3 shows a die apparatus 20 for cutting and forming sheet material 10 into a predefined configuration. The die apparatus can be used for hot or cold forming. The die apparatus can be used for forming sheet metal material, such as boron steel, aluminum alloy, magnesium alloy and titanium alloy, or it could be used for forming non-metal material, such as thermoplastics.

Figure 3 shows a tool apparatus 20 for cutting and forming sheet material 10 into a predefined configuration. The tool apparatus can be used for hot or cold forming. The tool apparatus can be used for forming sheet metal material, such as boron steel, aluminum alloy, magnesium alloy and titanium alloy, or it could be used for forming non-metal material, such as thermoplastics.

The tool apparatus 20 includes a die 22. The die 22 includes a surface 22a which has been molded to the shape of the predefined configuration. The tool apparatus 20 also includes sheet material holders 24. Further, the apparatus includes a punch 26. The punch 26 may also have a surface 26a molded to the shape of the predefined configuration.

The tool apparatus includes cutters 28, which take the form of blades. The apparatus includes two sets of opposing cutters 28. Each set of cutters includes a cutter 28 on the die 22 and a cutter 28 on the sheet material holder 24. Other configurations of cutters could be used. For example, the cutters could be positioned on other components of the apparatus 20, such as the punch 26.

In use, the sheet material 10 can be held between the sheet material holders 24 and a surface of the die 22. Force is applied to the sheet material holders 24 to hold the sheet material 10 in place during forming. The die 22 and the blank holder 24 are moved together such that the sheet material 10 is held tightly between the die and the holder. With this movement the cutters 28 are brought into contact with the sheet material 10 and cut the sheet material 10.

The cutters 28 are positioned in a design such that when brought into contact with the sheet material 10 they cut along a profile 14 of the sheet material 10 to define a blank. The cut can be complete such that the blank is completely separated from the remaining sheet material; or incomplete such that the blank is not completely separated from the remaining sheet material.

The die 22 is moved towards the punch 26. Alternatively, the punch is moved towards the die, or the two are moved towards each other simultaneously. The punch moves through the plane of the sheet material 10 and forces the sheet material into the molded surface 22a of the die. The sheet material 10 takes the shape of the molded surface.

The final panel includes the blank 12 shaped to the predefined arraignment. The remaining sheet material 10 is discarded or recycled.

Different options for the cutters 28 are illustrated in Figures 4a to 4c. The cutters 28a in Figure 4a include one blade attached to the die apparatus. The single blade is forced into the sheet material from one side, cutting through the thickness of the sheet material 10. The blade is not long enough to cut completely through the thickness of the sheet material 10, such that the resulting cut is incomplete.

The cutter 28b in Figure 4b includes two blades. The blades each cut into the sheet material 10 from opposite sides of the sheet material. The blades do not meet in the middle of the sheet material, such that the cut is incomplete.

Figure 4c shows cutters 28c which have opposing angled faces to shear through the sheet metal material 10.

It will be apparent that other types of cutters can be used.

As explained above, the cut made by the cutters may be an incomplete cut. That is, the blank may not be completely separated from the remaining sheet material. When the sheet material is being formed, the blank is still attached, at least partially, to the remaining sheet material. The blank is drawn into the die and is changed from its original shape. The drawing process causes stresses in the material. The remaining sheet material 16 which is partially attached to the blank results in further stresses which would not be present if the blank was completely removed from the remaining sheet material 16 before forming.

During forming, the sheet material 10 is drawn in a drawing direction. The remaining sheet material 16 creates tensile stress and strain along the drawing direction which reduces any wrinkling in the sheet material. The strain experience by sheet material 10 during drawing is illustrated in Figure 5.

Radial strain £ _r occurs in the sheet material in the direction of drawing the material, as a result of the force of the drawing. The remaining sheet material which is still partially attached to the blank provides opposing radial strain ε _ΓΓ in the direction opposite to the direction of drawing. The opposing radial strain counteracts the radial strain such that the sheet material is less likely to buckle than if under the radial strain alone (as would be the case if the residual sheet material was not present).

Such a configuration also means that the blank holding force (the pressure applied to hold the blank in place during forming) can be reduced.

Thus incomplete cutting facilitates material flow during forming, reducing wrinkling and suppressing the initiation of buckling.

Because the blank is not completely separated from the remaining sheet material during forming, in addition, because the pressure between die surfaces and the blank is reduced by reducing the blank holding force, in hot forming the heat lost as the edges of the blank (referred to as the "flanges") is reduced. The flanges therefore maintain a higher temperature, enhancing the drawability of the sheet material. The type of incomplete cut affects the stress state of the sheet material during drawing. Therefore different types of incomplete cut can be used to tune the material flow.

An incomplete cut can be made through the thickness of the material. Alternatively, an incomplete cut could be made by a series of complete cuts through the thickness of the sheet material along the boundary 14 of the blank 12. The complete cuts can be separated by intact sections of the boundary which have not been cut.

Dependent on the material, the forming process and the shape of the final panel, the type of incomplete cut at different sections of the boundary can be customized, as illustrate in Figure 6.

The boundary 14 has a first section 14a with a first type of incomplete cut. This incomplete cut could be, for example, a cut from one side of the sheet material 30% through the thickness of the sheet material.

The boundary has a second section 14b with a complete cut, wherein for that section, the blank is completely removed from the remaining sheet material.

The boundary has a third section 14c with a second type of incomplete cut. This could be, for example, a series of small complete cuts made along the boundary spaced 3 mm apart.

Varying the type of incomplete cut could include: varying the percentage cut through the thickness of the sheet material, varying from which side of the sheet material the cut is made, cutting from both sides of the sheet material, completing a number of complete cuts spaced along the boundary, varying the diameter of the number of complete cuts, varying spacing between the complete cuts, and any other conceivable change.

The profile of cutting pattern along the boundary is designed to tune the material flow during forming.

As the sheet metal material is drawn into the die, the blank is pulled away from the remaining material along the boundary. Once the forming process is complete, the blank is completely separated from the remaining material. The remaining material can be discarded or recycled. The blank has been shaped into the predefined arrangement to form the final panel.

Thus there is provided a method for preparing a sheet metal material for forming, and a method of cutting and forming sheet material into a final panel. It will be understood that the above description is of specific embodiments by way of example only and that many modifications and alterations will be within the skilled person's reach and are intended to be covered by the scope of the appendant claims.